The present invention relates to printers for digital data processing systems, and more particularly relates to ink-jet type printers.
A great diversity of different printing techniques for use with printers in digital computer systems have been shown. Very often, printing for digital computers is accomplished by creating a dot-matrix image of picture-element (pixel) data stored in the form of binary data in a computer. Examples of different dot-matrix printers include conventional multi-pin electro-mechanical impact printers, thermal process printers, electro-photographic printers, and more recently, laser printers and ink-jet printers. The quality of a printed image produced by such devices is frequently measured in terms of the "resolution" of the image, which in turn depends upon the minimum size and overall number of pixels per unit of area which can be produced with a particular printing process.
In addition to resolution, other useful considerations in the assessment of a printing system include the speed with which a printer can produce images, the noise made by the printer during printing, the cost of the printer, the size of the printer, the serviceability of the printer, the ease of use of the printer, the type of imaging agent used by the printer, the projected life-expectancy of the printer, and the special requirements on printing and printing materials imposed by the printer. Often, the design of printing systems represent a trade-off between certain of the above considerations. For example, while standard multi-pin impact-type dot matrix printers tend to be very fast, they also have limited resolution, on the order of one-hundred dots per inch or so, and can be very noisy during operation. Laser printers, on the other hand, are typically very fast and quiet, and have typical resolutions of three hundred dots per inch or greater; laser printers are also usually very expensive, however, their printing and paper handling mechanisms are quite complex, and they are sometimes quite large. Thermal printers are inexpensive, quiet, and fast, but permit printing only on specially prepared thermal material.
Ink-jet printers produce a pixel dot-matrix image by depositing very small droplets of liquid ink onto a printing surface. Ink-jet type printers are known to be fast and quiet, and may be somewhat less expensive than laser printers; the resolution of ink-jet printers is also typically very good, but can suffer according to the types of ink and paper used for printing. For example, some compositions of liquid ink are susceptible to "bleeding" in highly porous papers, and some glossy papers are not capable of retaining a liquid-ink image without smearing.
A number of different types of ink-jet printers have been shown in the prior art. In the so-called charged ink-droplet deflection type ink-jet printers, a series of electrically charged ink droplets are expelled from a nozzle and directed toward an absorbent printing surface, such as paper, through an electric field whose strength is modulated by the image to be reproduced; the individual charged ink-droplets are thus deflected selectively in accordance with the image as they pass through the modulated electric field. Ink which is deflected away from the paper during this process is typically collected in a trough or gutter, and possibly recirculated through the ink reservoir.
In another type of ink-jet printing, called thermal ink-jet printing, resistive heating is used to selectively vaporize liquid ink as it is expelled through an orifice. Ink not vaporized on expulsion is projected from the orifice to be received on the image surface. In still another type of ink-jet printing system, the so-called electro-pneumatic ink-jet printing system, controlled air-pressure gradients are employed to direct the flow of the liquid ink droplets toward the paper substrate. Other types of ink-jet printers are known which employ combinations or variations of the pneumatic and electrostatic techniques.
Often, an ink-jet print-head incorporates several independently controlled ink-ejecting nozzles, arranged such that several rows of pixels are printed with each pass of the print-head across the image surface. This allows more image area to be printed with each pass of the print-head over the printing surface, thus reducing the number of back-and-forth print-head passes per page and decreasing the printing time for each page.
Clearly, the aforementioned and other ink-jet type printing techniques involve delicate and complex electrical and mechanical operations which must occur in a precise, consistent and high-speed manner. Each involves the controlled propulsion of liquid ink from a reservoir to a substrate; each requires that the print-head's nozzles be moved back-and-forth with respect to the printing surface, possibly at very high rates. The motion of the print-head must be further coordinated with the motion of the printing surface being drawn past the imaging area of the print-head. Accordingly, ink-jet printers typically include complex plumbing and hydraulic systems and other very delicate or precisely adjusted mechanical components which must be maintained in an aligned configuration within the printer and which must be able to withstand the rigors of repeated movement and vibration encountered during printing.
During printing, the more delicate and intricate connections and precisely-aligned components within an ink-jet printer are not exposed to the user, and do not normally require adjustment or actuation by the user. For example, the plumbing system for conducting the liquid ink from a reservoir to an ink-jet or nozzle in an ink-jet printer is typically not accessed by the user during printing.
As with most printing systems, however, the supply of imaging agent (ink) in ink-jet printers must be periodically replenished; in particular, the ink reservoir in an ink-jet printer must occasionally either be refilled or replaced with a full reservoir; this usually involves the disconnection of various ink-conducting tubes within the printer, or the opening of the ink-containing reservoir itself. It is in such non-printing periods of operation that the deficiencies in the design of prior art ink-jet printers are most apparent.
When adding or replacing ink in an ink reservoir, there is usually a risk that some of the very high-viscosity fluid will be spilled. Furthermore, whenever the ink plumbing system is opened at any point, such as for removal of an empty reservoir, disconnection of hoses or other fittings, or removal of the print-head, that the user is exposed to small but significant amounts of uncontained ink, which may stain hands, clothing, or office furniture.
Attempts have been made in the prior art to provide ink-jet printers with "closed" ink systems in which the user is completely insulated from exposure to liquid ink. For example, some printers may employ a totally sealed unit containing the ink supply and ink-jet nozzles. The need for plumbing between the ink supply and the ink nozzles is eliminated by a direct, non-flexible connection therebetween. Replacement of the ink supply is accomplished by replacing the entire sealed unit, a "clean" operation in which the user is not exposed to liquid ink.
A significant disadvantage of the self-contained ink supply/print-head approach to preventing user-exposure is that the ink supply is coupled directly to the print head with a rigid connection, thus requiring the ink supply and plumbing system to be in motion with the print-head during printing. The weight of the ink supply increases the strain placed on the print-head motion mechanism, which must be able to move the print-head very rapidly back-and-forth across the printing area. The size of the ink reservoir must also be limited, so that the overall size of a printer utilizing the self-contained unit can be minimized. Accordingly, the amount of ink contained in the ink supply in such units is usually kept rather small; a typical self-contained ink supply/print-head unit may be capable of producing only about 500 printed pages before exhausting its ink supply.
Alternately, the ink reservoir in an ink-jet printer may be separate from the print-head, coupled thereto via a system of electrical, hydraulic and/or pneumatic connections. In this way, the print-head mass is kept to an absolute minimum, allowing for the fastest possible print-head transport speeds. The path of ink flow from the supply to the delivery system must be broken at some point, however, if the reservoir is to be replaced; disconnecting the print-head from the reservoir can thus be a complex and messy task.